Because no constraint is set for a wireless communications system and an operator to use an unlicensed spectrum, a case in which multiple operators of multiple types of communications systems occupy a same spectrum may occur. To avoid occurrence of the foregoing case, a rule for a wireless communications device to use an unlicensed spectrum needs to be formulated. For example, the European Telecommunications Standards Institute (ETSI) has released the ETSI EN 301 893, which stipulates a Listen Before Talk (LBT) rule for using an unlicensed spectrum. According to stipulations in the ETSI EN 301 893, the wireless communications device needs to use the LBT rule when using the unlicensed spectrum for communication, that is, the wireless communications device first monitors whether a channel on the unlicensed spectrum is idle, and if the channel is idle, the channel may be used. However, time of occupying the channel is limited. The wireless communications device may perform clear channel assessment (CCA) by means of energy detection and/or signal parsing, so as to determine whether the monitored channel is idle.
In a Long Term Evolution (LTE) system, if an LTE device (that is, a communications device in the LTE system) can be applied to an unlicensed spectrum, not only an unlicensed spectrum resource may be effectively used, but also more effective wireless access may be provided, and an increasing demand of mobile broadband services may be met. However, currently, a minimum time unit of data scheduling and data transmission of the LTE device is one subframe, and therefore, even if the LTE device detects that the unlicensed spectrum is idle or the channel on the unlicensed spectrum is idle, data can be transmitted on the unlicensed spectrum only after a subframe boundary moment arrives. Before arrival of the subframe boundary moment for which the LTE device waits, the LTE device cannot perform data transmission on the unlicensed spectrum, and therefore, the unlicensed spectrum is actually still in an idle state. In this case, another device (for example, a WiFi device) may occupy the unlicensed spectrum during the waiting time of the LTE device.
To prevent another device from occupying, during the waiting time of the LTE device, an available unlicensed spectrum detected by the LTE device, after determining that the monitored channel on the unlicensed spectrum is available, and before starting to send data, the LTE device may send a preemption signal (Padding in English) on the unlicensed spectrum. By using this method, another device may be prevented from occupying, during the waiting time of the LTE device, the idle unlicensed spectrum detected by the LTE device. However, if the padding is not properly designed, use of the unlicensed spectrum may be interfered with between LTE devices, and consequently the LTE device cannot use the unlicensed spectrum effectively, and utilization of the unlicensed spectrum is reduced.